If I already bought a halb-tax card and I want to buy a GA now, I heard that they will give me some kind of reimbursement for the halb-tax that I bought before. Has anyone done this or know how much this is or how it's calculated? Thanks.

can confirm, but cannot remember how it was calculated ... ask your friendly local SBB ticket counter ?

Also, another side question:

Does this GA cover local buses and trams as well in all cities? I can't seem to find a document stating what this GA covers exactly or doesn't cover.

What a GA covers:

http://mct.sbb.ch/mct/general-abo-uebersichtskarte.pdf

Take a look at this map. If the city/town is marked with a red dot with white circle around then the local transport network is included in the GA, I think pretty much all towns are.

Thanks! So if I'm reading it correctly, anywhere with the "bullseye" sign, I can use the local bus and tram system, correct?

Here's another train related question (that might deserve its own thread but it seems vaguely related to this one) - over the next 3 months I will be doing alot of travelling between Zurich and Basel (20+ day return trips) and I currently have a halb-tax card. Would it be cheaper for me to just stick with this or is it possible to buy a one or three month ticket?

u have 3 options:

ZH-BS monthly ticket
Yearly GA paid monthly by direct debit that u can cancel after 4 months (pro-rata refund on halbtax)
Buy a monthly GA with your halbtax for 350 p.m approx

again yourlocal friendly SBB counter can advise on prices....decision is urs.

That's right. The GA is a wonderful thing to have in your pocket! Depends on how long you have had the half price card as to how much they will refund. I bought a monthly season ticket and then the next day decided to upgrade to a GA and I got the whole of the season ticket price back.